1. Field of the Invention
This invention relates to an apparatus for collimating a laser beam, and, in particular, to an apparatus for collimating a laser beam emitted from a semiconductor laser device.
2. Description of the Prior Art
A semiconductor laser is well known in the art and widely used as a source for emitting a laser beam in various devices. However, since a laser beam emitted from a semiconductor laser is divergent, it is common practice to use a collimator lens for converting such a divergent beam into a collimated beam before use. For this reason, when a semiconductor laser is to be used as a light source, it is normally used in combination with a collimator lens and a holding member for holding the semiconductor laser and the collimator lens as spaced apart over a predetermined distance. Thus, a laser beam collimating apparatus normally includes three elements: a semiconductor laser, a collimator lens, and a holding member for holding the laser and the lens as spaced apart over a predetermined distance.
However, there are two factors which could drive the laser beam coming out of the collimator lens out of being collimated in such a laser beam collimating apparatus. The first factor is a temperature variation which would cause the holding member to thermally expand or contract. If this happens, since the holding member determines the distance between the collimator lens and the semiconductor laser, the distance between the semniconductor laser and the collimator lens varies so that the collimator lens becomes incapable of properly collimating the laser beam emitted from the semiconductor laser. The second factor resides in the fact that the oscillating wavelength of the laser beam emitted from the semiconductor laser varies with temperature. When the oscillating wavelength of the laser beam varies, due to the dispersion characteristic (i.e., the property of variations of the refractive index depending on the wavelength) of the glass material forming the collimator lens, the focal distance of the collimator lens changes substantially, thereby driving the laser beam output from the collimator lens out of being collimated.
It is true that, in order to cope with the first factor, it has been proposed, as disclosed in the Japanese Patent Laid-open Pub. No. 59-15206, to construct the holding member in a composite structure using several materials different in linear expansion coefficient such that the thermal variations in length in these various materials cancel out in total, thereby maintaining a constant distance between the semiconductor laser and the collimator lens. However, since this approach requires the holding member to have a composite structure, the overall structure becomes rather complicated and thus difficulty exists in manufacture. On the other hand, as a measure to cope with the second factor, it is conceivable to control the temperature at the semiconductor laser so as to keep it constant; however, such a scheme also requires a rather complicated temperature control mechanism which necessarily makes the overall structure complicated and larger in size.